https://leetcode.cn/problems/peak-index-in-a-mountain-array/description/
class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) {
        int left=0,right=arr.size()-1;
        while(left<right)
        {
            int mid=left+(right-left+1)/2;
            if(arr[mid]>=arr[mid-1]) //左边
                left=mid;
            else 
                right=mid-1;
        }
        return right;
    }
};